The use of low-mass tips as cathodes or electron sources for imaging purposes is known in the prior art. Field emission tips are commonly used, for example, in scanning electron microscopes because they provide a high brightness source. The electron optics which are used with such tips are typically relatively large discrete components with length scales on the order of centimeters.
More recently, the advantages of compact electron optics have begun to be appreciated. The advantages are at least two-fold. First, as the size of the optics is scaled down, the focal length also scales, as do the aberrations in the electron lenses. With smaller aberrations, a smaller spot size and higher current density are possible. Second, a small, low-mass electron optics system can be mechanically scanned, reducing the need for high voltage deflection plates to alter the electrons' trajectory and allowing new applications.
One such example is described in U.S. Pat. No. 4,760,567 to Crewe. This patent describes a compact, low-mass electron beam head for use in a high-density data storage device. In this system a field emission cathode is employed to write data on a rotating disk. An electrostatic focus lens is used to position the electron beam on the disk. The method of reading is not fully described. In particular, it is not specified how a read head would be positioned so as not to interfere with either the disk or the write head.
Recent advances in photolithography have made even more compact electron optics possible. Recently, Chang et al. have described the use of a scanning tunneling microscope, controlled field emission tip in conjunction with a microlens to form a high brightness electron source. The microlens is disclosed in various configurations. One arrangement that is non-focusing comprises a bias plane with a micro aperture that is positioned beneath the field emission tip. Another configuration, which can act as a focusing lens, includes a double conductive plane separated by an insulating layer; and a third includes three or more conductive planes separated by insulating layers. Those microlens structures enable the physical removal of the field emission electron source from the immediate vicinity of the sample's surface (as would be the case in a scanning tunneling microscope), but still enable a highly focused beam to impinge on the sample's surface. See "A Novel Scanning Tunneling Microscope Controlled Field Emission Microlens Electron Source" and "Electron Optical Performance of A Scanning Tunneling Microscope Control Field Emission Microlens System" Chang et al., Journal of Vacuum Science Technology, Vol. B7 No. 6, Nov/Dec 1989 pages 1851-1861, and Chang et al., Journal of Vacuum Science and Technology, B8, No. 6, Nov./Dec. 1990, page 1702.
The Chang et al microlens system constitutes a general purpose high-brightness electron source which can be used in place of a conventional electron beam column, for example for electron beam lithography. It does not, by itself, constitute an imaging system. It may be used for imaging, however, when combined with a detector of secondary electrons. Secondary electrons are emitted from a surface when electrons from an electron beam source, such as a microlens system, impinge on the surface with sufficient energy. The direction and degree of secondary electron emission depends strongly on the surface geometry. Emission of high-energy backscattered electrons can occur as well, depending on the surface material properties. A conventional scanning electron microscope (SEM) detects secondary electrons to generate its image. Material contrast can also be obtained using backscattered electrons.
The concept of using secondary electron detection in an electron beam memory device has been disclosed in a number of patents.
In U.S. Pat. No. 3,750,117 to Chen et al., an electron beam addressable memory is described where information is stored at deformations in a thin film. Read out is accomplished by scanning the film with an electron beam and determining the stored information by variations in secondary electron emission yield due to deformations in the film. A similar type of beam-accessed memory system is described by Nagao et al. in U.S. Pat. No. 4,748,592. It, too, employs emitted secondary electrons as indications of stored data in the storage media. Secondary electron detection has also been employed in field-emission scanning Auger electron microscopy, e.g. see U.S. Pat. No. 4,698,502 of Bednorz et al.
Other prior art that speaks to electron beam imaging systems can be found in the following:
U.S. Pat. No. 4,785,189 to Wells, U.S. Pat. No. 4,962,480 to Ooumi et al., U.S. Pat. No. 4,004,182 to Nixon, U.S. Pat. No. 4,896,045 to Okunuki et al., U.S. Pat. No. 4,823,004 to Kaiser et al., U.S. Pat. No. 4,343,993 to Binnig et al., U.S. Pat. No. 4,396,996 to Oldham, U.S. Pat. No. 4,575,822 to Quate, U.S. Pat. No. 4,785,437 to Dransfeld, U.S. Pat. No. 4,829,507 and U.S. Pat. No. 4,907,195 to Kazan et al., U.S. Pat. No. 4,968,888 to Appelhans, et al., U.S. Pat. No. 4,829,177 to Hirsch, U.S. Pat. No. 4,831,614 to Duerig et al. U.S. Pat. No. 4,791,301 to Nozue et al., U.S. Pat. No. 4,835,385 to Kato, U.S. Pat. No. 4,871,912 to Kokubo et al, U.S. Pat. No. 4,874,945 to Ohi, U.S. Pat. No. 4,954,704 to Ellings et al., U.S. Pat. No. 4,861,990 to Coley, U.S. Pat. No. 4,958,074 to Wolf et al., U.S. Pat. No. 4,902,892 to Okayama et al., and Re. 32,457 to Matey.
Disclosures of a similar nature can be found in the following Japanese published Patent Applications: 1-159,954 (A) to Oi, 62-31932(A) to Hirai, 62-137835(A) to Morizumi, 62-143355(A) to Fukuhara, 63-952(A) to Okubo, 63-12,148(A) to Tsukajima, 63-265,105(A) to Tamura, 1-287404(A) to Nishikawa and Kokai No. 54-105440 to Maebotoke.
Other published references generally describing electron beam writing and field emission tip electron beam generation can be found in "The Topografiner: An Instrument For Measuring Surface Microtopography" by Young et al., Review of Scientific Instruments, Vol. 43, No. 7, July 1972 pages 999-1011; "A High Density Data Storage System Based on Electron Beam Writing" by Berger et al., Inst. Phys. Conf. Serial No. 90: Chapter 4 presented at EMAG 87, Manchester, 8-9 Sept. 1987, pages 93-96; "Scanning Tunnelling Microscope Instrumentation", Kuk et al., Review of Scientific Instruments, Vol. 60, No. 2, Feb. 1989, pages 165-180 and "Magnetic Information Storage Apparatus" by Kump et al, IBM Technical Disclosure Bulletin, Vol. 9, No. 11, April 1967 page 1601. Published references describing microfabrication of field emitter tips can be found in "Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones" by Spindt et al., Journal of Applied Physics, Vol. 47, No. 12 Dec. 1976 page 5248, and "Formation of Silicon Tips with &lt;1 nm Radius" by Marcus et al., Applied Physics Letters, Vol. 56, No. 3 Jan. 1990 page 236.
It is therefore an object of this invention to provide an electron beam imaging system which employs a high brightness electron source, but uses secondary electron detection for imaging.
It is another object of this invention to provide an electron beam imaging system with an improved, integrated secondary electron detector.